The present invention generally relates to perpendicular magnetic recording medium, manufacturing method thereof and further to a magnetic recording apparatus, and more particularly to a perpendicular magnetic recording medium having a magnetic layer in which magnetic particles are isolated by a non-magnetic material.
Magnetic recording apparatus such as hard disk apparatus has the feature of low memory cost per bit and is capable of realizing large storage capacity. Thus, magnetic recording apparatuses are used extensively in personal computers, and the like, as digital signal recording apparatus. On the other hand, hard disk apparatuses are used also in various audio-visual applications, and there is a skyrocketing increase of demand for hard disk apparatuses. On the other hand, in relation to the purpose of recording video signals, there exist demands for hard disk apparatuses of further increased storage capacity.
In order to attain the desired increase of recording capacity and decrease of cost in magnetic recording apparatuses at the same time, it is effective to increase the recording density of the recording medium. With increase of recording medium, it becomes possible to reduce the number of the recording media in the recording apparatus, while this leads to decrease of the number of magnetic heads. As a result of decrease of the number of components, it becomes possible to decrease the cost of the magnetic recording apparatus.
For the improvement of recording density of magnetic recording media, it is necessary to improve the resolution and improve the signal-to-noise ratio (S/N ratio) by reducing noise. This reducing of noise has been proceeded conventionally by decreasing the grain diameter of the magnetic particles constituting the recording layer and further isolating the magnetic particles magnetically in the magnetic recording medium.
Patent Reference 1
                Japanese Laid-Open Patent Application 2003-217107Patent Reference 2        Japanese Laid-Open Patent Application 2003-346334Patent Reference 3        Japanese Laid-Open Patent Application 2005-353256Patent Reference 4        Japanese Laid-Open Patent Application 2006-309919        
Generally, a perpendicular magnetic recording medium has a construction of laminating a recording layer on a soft magnetic backing layer of a soft magnetic material formed on a substrate. A recording layer is generally formed of a CoCr alloy and is formed by sputtering the CoCr alloy while heating the substrate. According to such a process, it becomes possible to form magnetic particles of the CoCr alloy of Co-rich composition in the state non-magnetic Cr is segregated at the grain boundary of the magnetic particles. Thereby, it becomes possible to isolate the magnetic particles magnetically with each other.
On the other hand, the soft-magnetic backing layer is the layer used for providing a path of the magnetic flux returning to the magnetic head. Thus, when a crystalline state material is used for the soft-magnetic backing layer, there arises a problem of spike noise formation as a result of formation of magnetic domains. Thus, it is generally practiced to form the soft-magnetic backing layer by using amorphous state material or microcrystalline state material, in which formation of magnetic domains is not likely. However, with such a construction, there is imposed a limitation with regard to the annealing temperature at the time of formation of the recording layer in viewpoint of avoiding crystallization of the soft magnetic backing layer. Thus, there can be a case in which the segregation of Cr does not proceed sufficiently, resulting in incomplete magnetic isolation of the magnetic particles.
Meanwhile, there is a proposal of a recording layer capable of promoting isolation of the magnetic particles and at the same time not necessitating high temperature annealing, wherein the recording layer of this proposal has the construction of forming magnetic particles of CoCr alloy in a non-magnetic matrix of SiO2 in mutually isolated state. Further, there is a proposal of the technology of forming a Ru film as the foundation layer of the recording layer and causing the growth of the magnetic particles in the recording layer in the direction perpendicular to the substrate surface, such that there is formed a columnar structure in which the c-axes of the magnetic crystals are aligned in the recording layer in the direction perpendicular to the substrate surface and such that the magnetic particles are grown with generally equal interval in the magnetic layer. Reference should be made to Patent References 1-2.
Meanwhile, in the case a Ru film is merely formed as the foundation of the recording layer, it should be noted that the magnetic particles grow merely on the surface of the crystal grains of the Ru film. Thus, depending on the size or arrangement of the crystal particles, there can be a case in which magnetic particles are coupled with each other and the desired isolation of the magnetic particles is not attained. Further, there is a concern that there occurs spreading of grain diameter distribution for the magnetic particles, resulting in increase of the medium noise.
When to form the adjacent magnetic particles with generally uniform interval, on the other hand, there is a need of providing a seed layer at the underside of the Ru film, while such a construction requires a lamination of a plurality of seed layers. Thus, there arises a problem of increase of thickness of the seed layer. Further, when such a thick seed layer is interposed between the recording layer and the soft-magnetic backing layer, there is caused increase of distance between the soft-magnetic backing layer and the recording layer, while this leads to the problem of increase of head magnetic field necessary at the time of recording. Further, there occurs spreading of the head magnetic field, while this leads to the problem of side-erasing of information in which the information on the adjacent track is erased because of the spread head magnetic field.
In relation to the foregoing problems, the inventor of the present invention has proposed previously a perpendicular magnetic recording medium that uses a recording layer having a columnar granular structure, a manufacturing method thereof, and a magnetic recording apparatus, wherein the size distribution of the magnetic particles is improved and the magnetic particles are distributed uniformly. Reference should be made to Patent Reference 3.
FIGS. 1 and 2 show the construction of a perpendicular magnetic recording medium 60 of the foregoing proposal as a related art of the present invention.
Referring to FIG. 1, the magnetic recording medium 60 includes a soft-magnetic backing layer 2 formed on a substrate 1, wherein the soft-magnetic backing layer 2 carries thereon a grain growth nucleation layer 61 via an orientation control layer 3 that controls the orientation of the magnetic particles in the recording layer to be formed on the soft-magnetic backing layer 2. For the orientation control layer 3, an amorphous layer of Ta may be used, for example.
Referring to FIG. 2, the grain growth nucleation layer 61 is formed of island patterns 61a of Pt, or the like, formed on the orientation control layer 3 uniformly and with generally equal interval, and there is formed a first foundation layer 21 of Ru crystal grains 21a of generally identical grain diameter, wherein the Ru crystal grains 21a are grown respectively in correspondence to the island patterns 61a. 
Further, on the first foundation layer 21, there is formed a second foundation layer 4 of Ru as shown in FIG. 1, wherein the second foundation layer 4 is formed as an assembly of columnar Ru crystal grains 4a respectively grown from the Ru crystal grains 21a in the first foundation layer 21. Here, it should be noted that the Ru columnar crystal grains 4a are grown epitaxially in the foundation layer 4 with respect to the Ru crystal grains 21a with a gap 4b. 
Further, there are formed, on the second foundation layer 4, columnar crystal grains 5a of CoCrPt grown epitaxially to the Ru crystal grains 4a in the second foundation layer 4, wherein the CoCrPt crystal grains 5a are isolated with each other by an SiO2 grain boundary phase 5b and constitute a recording layer 5. With such a construction, it should be noted that the CoCrPt crystal grains are oriented such that the c axes thereof are aligned perpendicularly to the substrate 1 and have an easy axis of magnetization in the direction perpendicular to the substrate 1.
Further, the recording layer 5 is covered with a protective film 6.
Thus, with the perpendicular recording medium 60 of FIGS. 1 and 2, the magnetic crystal grains 5a each having the easy axis of magnetization aligned perpendicularly to the substrate 1 are disposed via the non-magnetic grain boundary phase 5b, and thus, isolation of the magnetic particles is promoted. Thereby, the perpendicular magnetic recording medium 60 shows the feature of reduced medium noise.
On the other hand, with the perpendicular recording medium 60 of FIGS. 1 and 2, there is a need of forming the island patterns 61a on the orientation control layer 3 with generally equal interval as shown in FIG. 2, while formation of such island patterns 61a requires a complex process such as mask process, and thus, the perpendicular magnetic recording medium 60 cannot avoid the problem of increase of manufacturing cost.